1. Field of Art
The present invention relates in general to method and apparatus for broaching an outer surface of a resin layer which covers an external surface of a part, and more particularly to improved method and apparatus for external broaching operations on such external resin layers of a workpiece, without breakage, chipping or flake-off of the broached resin layers.
2. Related Art Statement
In the art of finishing a resin-coated part to desired external shape and dimensions, it is known to use a broaching method or apparatus for cutting an outer peripheral surface of a resin layer which covers the part. Such external broaching method and apparatus encounter an inconvenience that the resin layer is subject to local breakage, chipping, flake-off or other defects at or near the end of a broaching cut or pass in the direction along the resin layer to be broached. In broaching a resin-coated part 64 shown in FIG. 6, for example, the reaction force of a resin layer 62, which resists a shearing force exerted by a broach 60, decreases as the broach 60 approaches the end of its broaching movement toward the lower end of the part 64. Then, the amount of deformation of the resin layer 62 over the unbroached length to provide the reaction force resisting the shearing force increases as the unbroached length decreases. As a result, the lower end or corner portion of the resin layer 62 is broken or chipped, and the resin layer 62 is more or less separated or peeled off the surface of the part 64, as indicated in FIGS. 7 and 8.
Examples of resin-coated parts includes a rotor assembly 66 of a Roots pump shown in FIG. 9, which includes a rotor 65 made of a metallic material and coated with the resin layer 62. In this case, the chipping of the resin layer 62 at its corner portion during the broaching cut results in a gap 70 being formed between the inner surface of a pump housing 68 and the rotor assembly 66, whereby there arises a reverse flow of a fluid through the gap 70, which deteriorates the pumping capability of the Roots pump.